The developmental importance of the gonadal hormones on sexual differentiation of behavior and reproductive (neuroendocrine) function in mammals is well documented and has led to the concept that the presence or absence of androgen is the determining factor in the gender-specific patterns of neural organization. The cellular mechanisms underlying sexual differentiation of the brain as well as the very neural substrate(s) involved, however, remain essentially unknown. Cellular aspects of sexual differentiation will be studied in organotypic cultures of some regions of the newborn rodent brain implicated in this process - the hypothalamus preoptic area, and in other areas serving as regional controls - cerebellum, cerebral cortex, in order to better define the role(s) of the gonadal steroids during neuro-genesis. The morphological parameters of neural differentiation will be studied in both living and stained preparations at the light and electron microscopic levels and correlated with biochemical studies of the steroid receptors and steroid effects. These studies are designed to clarify the following hypotheses: (1) that the gonadal hormones exert a morphogenetic effect on the developing central nervous system (CNS) during "critical" or steroid-sensitive periods of neural differentiation; (2) that steroid responsiveness is characterized by the selective acceleration and enhancement of neuritic development which influence the establishment of neuronal interactions and the patterns of neural organization; (3) that the developmental steroid effects are mediated through high-affinity specific receptors which are present during the ontogeny of the various CNS regions at a time corresponding to their critical period; (4) that steroid responsiveness is a general developmental property of the CNS distinct from the steroid-sensitivity of the adult and may not be limited to the phenomenon of sexual differentiation; and (5) that estrogen per se may have an important morphogenetic role throughout the CNS of both genders during ontogeny so that male and female patterns of sexual differentiation of the brain may both require active induction by steroid. Such studies offer a new and unique approach to an understanding of the developmental aspects of the neural regulation of sexual behavior, of sexually dimorphic non-mating behavior and of pituitary activity, as well as of the organizational mechanisms by which interactions between the developing neural substrate and the gonadal hormones may induce, regulate or modify the basic developmental processes.